DECAPOD CRUSTACEANS OF THE WEST INDIES 101 



In some males, the colors are predominantly blue and yellow with 

 the red areas described above replaced by shades of yellow. In those 

 shrimp adapted to hght backgrounds, the color is mostly pale tan 

 with pale yellow and light brown. The second pereiopod is lavender 

 below, olive green above with purple tubercles and fingers, the latter 

 with white tips; the basal portion of the dactyl is bright orange. The 

 second through the fifth pereiopods are bluish with pink bands. 



Most females are not so richly colored, nearly always lacking the 

 strong contrasts between the shining black and brilliant reds and 

 yellows, and seldom do they exhibit the rich blue-to-purple coloration 

 of the males. 



Three male shrimp that had been collected during the morning were 

 retained throughout the remainder of the day and evening in a light 

 blue plastic bucket. At approximately 10:00 p.m., about an hour 

 before the light was extinguished, two of them were placed in a dark 

 purplish red bucket. At 7:00 a.m. on the following morning (still 

 almost dark where the shrimp were maintained), the two that had 

 been moved to the dark bucket were very dark purplish red with 

 little yellow evident, but the one in the light blue bucket was trans- 

 lucent pale tan. This seems of particular interest because the shrimp 

 were able to become adapted to their respective backgrounds after a 

 very short exposure to very dim light. 



Material examined. — The Dominican collections contain 149 

 males (carapace lengths 2.9-29.7 mm), 106 females (cl 3.0-25.3 mm) 

 including 19 with eggs (cl 8.7-19.0 mm), and 22 juveniles (cl 2.0-2.8 

 mm). The smallest recognizable male, in which the appendix mascu- 

 Hna on the second pleopod is no more than a bud, has a carapace 

 length of 2.9 mm, but that appendix may be less than fuUy developed 

 at a carapace length of more than 6 mm. 



Ecological notes. — Macrobrachium crenulatum occurs most abun- 

 dantly in pools in the small feeder streams cascading from the moun- 

 tains but is also at home in small, flowing drainage ditches and in the 

 shallow rocky areas of larger streams. In the pools, this shrimp may 

 be seen moving from place to place at almost any time of the day. 

 If a person slowly approaches a pool, the shrimp may often be seen 

 moving about apparently aimlessly or resting in the open with their 

 antennae gently waving to and fro. The larger individuals, however, 

 are for the most part at least partially concealed beneath a stone or 

 under debris. In and immediately upstream and downstream from 

 pools in Mannet's Gutter, this is undoubtedly the most abundant 

 shrimp. Unhke M. carcinus, a single small pool may support a dozen 

 or more M. crenulatum. In the smaller pools there may be only a 

 single large male; however, in the larger ones, there are often three 

 or four of them. 



